1. Field of the Invention
The present invention relates to implantable knee joint prostheses for replacement of the engaging surfaces of the femur and tibia of a dysfunctional human knee joint, and particularly to a modular system for allowing the physician to select and connect together appropriately sized components which provide posterior stabilization of the knee while allowing essentially free translation, rotation and angulation through the range of articulation of the knee, while still avoiding dislocation.
2. Brief Description of the Prior Art
Implantable knee prostheses for diseased and/or damaged human knees have typically employed three components, namely, the femoral, tibial and patellar replacement components. These components are surgically implanted in patients whose knee joints have deteriorated resulting from injury, arthritic disease processes or failed previous prostheses. Surgical procedures involve the trimming of the femur and tibial joint surfaces to accommodate the appropriately sized components of the prosthesis. Typically, the surgeon is required to have available a number of differently sized and configured knee joint prostheses on hand in order to make a judgment as to which prosthesis to employ in the surgical procedure with the particular patient. In the past, the components of the knee prosthesis have been factory assembled or, as described in U.S. Pat. Nos. 4,936,853 and 4,822,366, separate modular component parts of the complete knee prosthesis are maintained at hand for assembly in the operating room at the time of surgical implantation.
The selection of a particular prosthesis for a knee joint is based largely on the condition of the patient's knee and depends upon whether there is little disease or injury of the bones, ligaments and other tissues or whether there are severely damaged joints which require implantation of components that have greater inherent stability and augments for enhancing component fixation.
The knee joint prosthesis described and shown in U.S. Pat. No. 3,837,009 is exemplary of a posterior stabilized prosthesis. It includes a post that extends upwardly from the tibial component into a slot between the femoral condyles in the femoral component. An axle affixed to the femoral component and passes through an elongated hole of carefully designed shape and size in the post. Forces that are transmitted between the axle and hole can be very large, and they act at a relatively great distance from the tibial plateau; as a result, there is a fairly high degree of risk of the tibial component becoming dislodged from the tibia. A number of other known knee joint prostheses of the type that are designed to impart stability to the knee joint by mechanical action are similarly subject to failure. In some cases, the implantation of the prosthesis requires removal of a considerable amount of bone, and failure may include fracture of the remaining, weakened bone.
Between the two extremes requiring simple, small components (see, for example, U.S. Pat. No. 3,774,244) on the one hand and prostheses constructed to have inherent mechanical stability on the other is a relatively wide range of damaged knee joints in which most of the articulating surface at the joint must be replaced but most ligaments and other tissues that provide stability in the anatomical knee joint are undamaged or can be repaired so that essentially normal restraint and control of joint function are provided by remaining soft anatomical elements. Prostheses constructed for replacement of substantially all articulating surfaces of the bones at the joint are often referred to as "total" condylar joint prostheses, and a number of total knee joint prostheses of the condylar type have been proposed (see, for example, U.S. Pat. Nos. 3,748,662, 3,816,855 and 3,869,729).
Generally, a total knee joint prosthesis of the condylar replacement type includes a tibial component having a platform portion which replaces all of the superior surface of the tibial plateau and substitutes for the anatomical tibial condylar surfaces. The femoral component has laterally spaced-apart condylar portions joined by an intercondylar bridge and a patellar surface and thus affords replacement of substantially all of the surfaces of the femur that engage the tibia and patella. Whether or not the cruciate ligaments are retained in a condylar replacement prosthesis depends on the design.
The tibial component typically includes a tibial tray and stem (which may be integral or modular) for surgical attachment to the tibia and a modular articulating surface member (also referred to as a tibial insert) that is attached by a variety of mechanisms to the superior surface of the metal tibial tray. The tibial insert is constructed of a ultra high molecular weight polyethylene, a known polymer used for prosthesis bearing surfaces, and is shaped from a pair of laterally spaced concavities to receive the pair of laterally spaced-apart femoral condylar portions. The condylar portions of the femoral component are smoothly curved in the anterior-posterior direction generally to match in lateral profile the shapes of the condylar surfaces of the femur and smoothly convexly curved in all cross sections along their anterior-posterior extents.
Modular knee prostheses of this general description include both non-stabilized and posterior stabilized types. A non-stabilized modular knee prosthesis does not provide any substantial restraint against rotational and translational freedom of movement nor any mechanism to inhibit anterior-posterior dislocation on hyperextension or flexion and are exemplified by the above referenced '244 patent as well as the more recent U.S. Pat. No. 4,938,769. These total knee prostheses require only minor resection of bone for implantation, do not require severance of the posterior cruciate ligaments, and are suitable for relatively mild deterioration of the knee joint.
Posterior stabilized knee prostheses are used in posterior cruciate ligament deficient knee joints. They are characterized by an eminence extending superiorly between the parallel concavities of the tibial insert and into a recess or box-like intercondylar portion of the femoral component. The cruciate ligaments are normally severed, although the collateral ligaments and tendons remain intact.
The intercondylar recess is formed usually with parallel lateral side walls spaced-apart by anterior and posterior bridge members which may act as camming members for bearing against anterior and posterior camming surfaces of the eminence through virtually the entire range or a portion of the range of flexion to inhibit dislocation of the joint at hyper extension and flexion.
A number of posterior stabilized knee prostheses have been proposed and introduced for clinical use, including those disclosed in U.S. Pat. Nos. 4,209,861, 4,298,992, 4,213,209 and 5,007,933. The eminences and intercondylar portions disclosed in these patents have a variety of slows and dimensions to which greater or lesser degrees of restraint and freedom of relative movement are attributed. In the '861 patent, virtue is ascribed to virtually total contact of the posterior camming surface and camming member through the entire range of flexion, and the lateral side walls of the eminence and the recess are quite closely spaced to allow minimal translation or varus-valgus movement and virtually no rotational movement.
The '992 patent discloses an eminence and recess shaped to provide positive camming surface and member contact only after substantial flexure. The lateral side walls of the eminence and recess disclosed in the '992 patent appear to be parallel and closely spaced, permitting little if any lateral translation or rotation. The eminence and recess lateral side walls disclosed in the '209 patent are not parallel. The lateral walls of the eminence converge superiorly and away from the lateral side walls of the recess and also converge anteriorly.
The '209 patent shows the spaced concavities of a tibial articulating surface member being smoothly radiused with the rising side walls of the eminence to accept femoral condylar portions shaped to contact at least a portion of the side walls during articulation, rotation and translation. The degree of divergence between the side walls of the eminence and the walls of the recess superiorly limits the degree of lateral angulation and prevents extreme translation.
The '933 patent discloses an eminence and recess shaped to provide posterior camming surface and member contact from about 30.degree. through full flexion. The lateral side walls of both the eminence and recess are relatively closely spaced and the parallel portions of both tend to converge superiorly. As in the '209 patent, at least a portion of the lateral side walls of the eminence are angled to converge anteriorly. The angulation of the '933 patent is relatively small to provide relatively limited rotation of the femoral and tibial components within a relatively limited range of flexion in comparison with the configuration disclosed in the '209 patent.
The physiology of the normal knee is such that as the leg is flexed, the net force component tending to translate the femoral condyles anteriorly of the tibial insert (referred to sometimes as a shear force) increases. As mentioned above, forward displacement of the femur is resisted, in the case of posterior stabilized knee prostheses, by contact between the respective posterior bearing surfaces of the tibial eminence and the intercondylar recess. The posterior bearing surface of the femoral component shown in the '933 patent is sharply rounded and makes essentially point or line contact with the confronting bearing surface of the tibial eminence throughout the range of flexure of the leg. In somewhat similar fashion, the area of contact between the posterior bearing surfaces of the components shown in the '992, '861 and '209 patents appears to be essentially constant throughout the range of flexure in which these surfaces are in contact.
In conjunction with posterior stabilized knee prostheses having a pronounced eminence, the forces brought to bear against the eminence requires secure attachment of the tibial insert to the tibial tray. Various attachment mechanisms of tibial insert to tibial trays have been used in the past; e.g., the matching dovetail and slot "snap-lock" system of the AXIOM.TM. Total Knee System or the dovetails and locking pin of the CPD (Central Post Design) tibial components, both sold by the assignee of the present invention. In the context of a posterior stabilized knee prosthesis, such locking mechanisms have been disclosed in the '209 patent and in U.S. Pat. Nos. 4,714,474 and 5,007,933.
Thus a number of posterior stabilized knee prosthesis have been proposed that provide varying degrees of restrained to unrestrained relative anteroposterior, rotational and translational movement of the femoral and tibial components. A need remains for a knee joint prothesis that provides appropriate roll back and a high degree of rotation and angulation to mimic the freedom of movement of the normal knee while advantageously limiting anteroposterior dislocation (subluxation).